Proceedings of the Fourth Genoa Meeting on Hypertension, Diabetes, and Renal Diseases
Stem Cells and the Kidney: A New Therapeutic Tool?
Gianpaolo Zerbini*,
Lorenzo Piemonti,
Anna Maestroni*,
Giacomo Dell’Antonio and
Giuseppe Bianchi
* Renal Pathophysiology Unit, Division of Medicine, Section Nutrition-Metabolism, Telethon-Juvenile Diabetes Research Foundation Center for Cell Replacement, Human Pathology Division, and Division of Nephrology, Dialysis and Hypertension, San Raffaele Scientific Institute, Milan, Italy
Address correspondence to: Dr. Gianpaolo Zerbini, Unita’ di Fisiopatologia Renale, Istituto Scientifico San Raffaele, Via Olgettina 60, I-20132 Milano, Italy. Phone: +39-02-26432148; Fax: +39-02-26432914; E-mail g.zerbini{at}hsr.it
In the past few years, a number of studies have shown that stemcells can be found in virtually every organ of the adult organism.The kidney is not an exception, and resident stem cells havebeen identified both in the papilla and along the tubules. Ofinterest, kidney-bound stem cells have been identified alsoin the bone marrow. When injected, both resident and bone marrow–derivedstem cells are able to reach the injured renal tissue and, oncethere, to differentiate into renal cells. The evidence that,in humans, some of the acute and most of the chronic renal damageslead to ESRD suggests that in normal conditions, the reservoirof stem cells (considering both resident and bone marrow–derivedstem cells) is insufficient to allow a major renal regeneration.Probably the number of stem cells that are ready to intervenein an adult kidney are sufficient to compensate for the normalcell turnover but largely inadequate to counteract a major injury.This is confirmed further by the finding that, even by transplantinga syngenic bone marrow in rats with ablation of 5/6 of the renalfunction, it is not possible to increase the life expectancyof the animals. Altogether, this evidence suggests that, toclarify the potentiality of a stem cell therapy for renal diseases,experiments that aim to clarify the ideal concentration of stemcells to be injected and to identify the best way of administrationare needed.
Significant attention has been directed recently to study thepotentiality of stem cells in the treatment of a number of acuteand chronic diseases. Stem cells are undifferentiated cellsthat are characterized by a high degree of self-renewal anddifferentiation potential (1). Stem cells can be isolated froma variety of mammalian tissues and organs both during developmentand into adulthood. Among the characteristics of stem cellsis the capability of self-maintenance, their indefinite proliferativepotential, and the ability to generate many if not all of thedifferentiated cell types that are contained in an organ (2–4).In the presence of damage, these cells can replace the injuredones (5–7). Stem cells are responsible for the developmentand growth of different organs during embryogenesis and fortissue homeostasis and repair during adult life (8).
Although tissue plasticity during adulthood is manifest clearlyin some organs, such as the small intestine, the liver, andthe hematopoietic system, in some others, such as the centralnervous system, the possibility of self-renewal has been recognizedonly recently (9). In the kidney, tubules and glomeruli showa totally different plasticity. It is widely known from theclinical practice that, in most cases, tubules are able to regenerateeven after major damage (10), although postnatal glomerulogenesishas not been described in human. Probably for this reason, majoracute or chronic glomerular damage invariably leads to ESRD.
The recent identification of renal progenitor cells both insidethe kidney (11,12) and in the bone marrow (13–17) maypave the way toward the future regeneration of the damaged kidney.Appropriate functional experiments in animal models of renaldamage now are needed to clarify the therapeutic potential ofa stem cell approach in repairing a, so far, irreversibly damagedkidney.
Resident progenitor cells have been identified in the papillaof the adult murine kidney (11). In the papilla, stem cellsreside in niches and co-express mesenchymal and epithelial antigens;once injected under the renal capsule, they incorporate in renalparenchyma and tubules. More recently, renal progenitor cellshave been found also in the tubular fractions of the cortexof adult human kidney (12). These cells differentiate in vitroin epithelial and endothelial cells, and, once injected intravenouslyin SCID mice with an induced acute tubular injury, they localizein proximal and distal tubules. The resident progenitor cellsare potentially clinically useful because these cells, at differencewith multipotent stem cells, should be available immediatelyto repair the kidney and therefore could be of great help inthe treatment of acute renal injuries. However, it seems questionablethat a single adult progenitor cell may be able to repair allof the various parts of the kidney. It seems possible that,in the near future, more renal progenitor cells will be identifiedinside the kidney, each one committed to repair a differentcomponent.
A number of studies showed recently that bone marrow representsa reservoir of stem cells that are physiologically involvedin remodeling and repairing the kidney. Bone marrow can providecells that integrate into the kidney and differentiate intonew functional renal cells of a variety of types. There is evidenceof engraftment and differentiation of stem cells during normalrenal cellular turnover (13) and after acute and chronic damage(14–17). Irradiated female mice that received a transplantof male bone marrow formed renal tubular cells that containedthe male donor Y chromosome, suggesting that bone marrow cellscan migrate to the kidney and form tubular epithelium (13).
Several studies have reported the contribution of bone marrowstem cells in the repairing of damaged glomeruli. In particular,bone marrow cells were shown to differentiate into mesangialcells in murine recipients when the glomerulus was damaged byan antibody-mediated glomerulonephritis (16). Other studiesdocumented how bone marrow contributes to the maintenance andrepair of renal endothelium and interstitium (18,19). In themajority of the studies, the injected material was whole bonemarrow, meaning a mixture of multiple types of stem cells, includinghematopoietic stem cells, mesenchymal stem cells, multipotentadult progenitor cells, and side population cells.
Regarding the possibility that a specific stem cell type mightbe better than others for kidney repair, the literature is stillcontroversial: In some studies, hematopoietic stem cells wereshown to contribute to tubular epithelium repair (14), whereasin other models, only mesenchymal stem cells accelerated thestructural recovery of the kidney after injury and conferredtherapeutic benefit (16).
Can Renal Function Be Restored by Transplantation of Whole Bone Marrow?
Despite the large number of studies that aimed to identify renalstem cells, very few data are available concerning the possibilityto restore renal function by means of stem cell therapy. Toclarify whether an acute renal injury can benefit from a simultaneousinjection of stem cells, we recently performed a study in whicha classic 5/6 removal of renal function (20) was treated bya whole bone marrow transplant. This experiment was done inrats that maintained the native bone marrow.
As shown in Figure 1, 40 Milan normotensive strain rats wereincluded in the study. After removal of one kidney and clampingof two of the three afferent arterioles of the contralateralkidney, only approximately one sixth of the total renal functionwas left. Ten rats, used as controls, received an intravenous(vena cava) injection of 100 µl of serum-free minimalessential medium (Sigma, St. Louis, MO), 10 rats received 25million splenocytes that were obtained from syngenic donors(in 100 µL of minimal essential medium) as controls forcellular mass, 10 rats received 5 million whole bone marrowcells, and finally 10 rats received 25 million whole bone marrowcells. The rats were followed until death, and serum creatininewas measured on a weekly basis. As shown in Figure 2, the fourgroups of animals behaved in a similar way, and after an initialrecovery of renal function in the first 2 to 3 wk after surgery,creatinine levels increased progressively. Death rate as a resultof renal insufficiency also was similar in the various groups,and 3 mo after surgery, no animals were still alive. Renal massrapidly recovered after surgery, but this was mostly due totubular regeneration. New glomeruli could not be found in theregenerated area. Finally, the clamped area of the remnant kidneyunderwent necrosis and fibrosis, and no neovascularization couldbe appreciated inside this tissue.
Figure 1. Study design. Forty Milan normotensive rats were involved in the study. Removal of 5/6 of renal function was performed in each animal. Rats then were divided in four groups that underwent injection of minimal essential medium (control group), 25 x 106 splenocytes, 5 x 106 whole bone marrow cells, or 25 x 106 whole bone marrow cells.
Figure 2. Plasma creatinine levels in the four groups of rats considered for the study during the 18 wk of follow-up. The number of rats that were alive at each creatinine measurement is indicated at the bottom of the figure.
Our results are in touch with previous reports suggesting thatbone marrow engraftment in the presence of native bone marrowis very low (21), probably because of competition between thetwo components. The previous finding that, at difference withglomeruli, renal tubular component is able to regenerate alsowas confirmed (10). New experiments now are needed to clarifywhether less severe renal damage can be recovered using thesame approach; whether small focal damage can be treated successfullywith this kind of therapy; whether a direct injection of cellsinside the renal parenchyma can exert a better result; and,finally, whether, by injecting selectively stem cells or renalprogenitors rather than whole bone marrow, the final resultcan be improved.
Although some components of the kidney, such as the tubules,are able to regenerate spontaneously, some others, such as theglomeruli, cannot recover spontaneously from a major injury.This finding suggests that the reservoir of plastic cells thatare able to regenerate the glomeruli probably is enough forsupporting the normal cell turnover but insufficient to intervenein case of major tissue losses. The isolation and the expansionin vitro of the progenitors of the various kidney componentsalong with the definition of the correct "dosage" and administrationroute should, at least in principle, allow in a near futureidentification of the correct stem cell therapy for both acuteand chronic renal diseases.
Acknowledgments
We are indebted to Elena Minotti for technical assistance.
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Cell Replacement, 
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Abstract
Introduction
